1. Field of the Invention
This invention resides in the field of electroporation, a process for inserting exogenous molecular species into membranous structures by suspending the structures in a liquid solution of the exogenous species and applying an electric field to the suspension. In particular, this invention concerns apparatus used for high-throughput electroporation, which term is defined as electroporation performed in a multitude of cell suspensions either simultaneously or in rapid succession.
2. Description of the Prior Art
Electroporation is a technique that involves the use of an electric field to impregnate membranous structures such as living biological cells, liposomes, and vesicles with exogenous molecules. High-throughput electroporation allows a user to apply an electric field, i.e., to “shock,” multiple samples either simultaneously or automatically in sequence. High-throughput electroporation is practiced in a variety of procedures, notably experiments involving siRNA and research involving the use of cDNA libraries.
A high-conductivity buffer is used as the medium in which the exogenous species are dissolved and the membranous structures suspended during electroporation, and normal saline is commonly used since, in addition to presenting a relatively low resistance to an electric current, it offers the most favorable environment for the viability of most cells. In general, however, electroporators are limited by their resistance to electrical energy. Hence, multi-welled plates, such as those with a standard 96-well array, are typically shocked in sections, such as one eight-well bank at a time until all twelve banks have been shocked.
One manufacturer, BTX Instrument Division, Harvard Apparatus, Inc., Holliston, Mass., USA, offers a high-throughput electroporator designed to shock the contents of 96 wells of an 8×12 array. This electroporator is described in an International Patent Application published under the Patent Cooperation Treaty, Publication No. WO 2004/050866 A1, the contents of which are incorporated herein by reference. The well plates described in WO 2004/050866 A1 are made with rectangular wells with electrodes plated on the walls of each well. All of the electrodes on one side of a bank of eight wells (i.e., a column) are connected in common to plated traces along each bank through wire connections, and all electrodes of the other side of the wells in the same bank are likewise connected in common. Corresponding electrical connections exist in all twelve banks. Because of the low resistance of eight parallel wells, however, and the fact that the maximum capacitor available has a capacitance of about 3200 mfd, the simultaneous shocking of eight wells limits the maximum time constant to about 20 msec and the resistance load to about 6.25 ohms. In many protocols, the shocking of all 96 wells requires ten minutes. The supply of electric power to the plate is achieved by lowering the plate into a plate handler which makes electrical contact between electric leads in the handler and each of the electrodes in the plate and has internal drivers or relays that connect each of the twelve banks in sequence to an external power supply. Electric pulses are then delivered to the electrodes in sequence. Rectangular wells of the same size and spacing are used for both 96-well plates and 25-well plates, and all electrical leads and connections are located in the “solid substrate” that forms the base of the plate. This limits the use of the wells and deprives the electroporator of versatility.